Field of the Invention
The present invention relates to rotating electrical machines and to brushless machines which include at least three phase windings which are connected to each other.
Description of the Background Art
To avoid the known disadvantages of rotating electrical machines having mechanical commutator devices in the form of brushes, for example the formation of brush arcing and the problems of wear on the mechanical commutator device with corresponding losses of useful power, so-called brushless electrical machines were developed using the capabilities of power electronics, whose susceptibility to faults is much lower and whose wear is essentially determined by bearing wear and is thus very low.
In a brushless electric drive of this type, the rotor usually has permanent magnets, which move within a rotating magnetic field of the stator that is provided with phase windings. The windings of the stator are activated with the aid of power semiconductor switches, in particular with the aid of power transistors, in particular MOSFETs. The different phase windings of the stator are activated cyclically, a pulse-width modulation normally being used for activating the individual phase windings for the purpose of applying a precisely and quickly controllable current thereto.
A rotating electrical machine comprising multiple phase windings is known from WO2008/006745, which corresponds to US 20090230907, in which the activation is carried out by alternately supplying different voltage potentials to the phase winding terminals, a terminal potential being maintained constantly in time intervals at one of the phase windings. The electrical potentials at the remaining phase winding terminals may be dynamically controlled in such a way that corresponding current characteristics are generated therein. The supply of voltage potentials and the application of current to the remaining phase windings are sensibly carried out with the aid of pulse-width-modulated signals, which are switched back and forth between a lower voltage level and an upper voltage level at high frequency, the current intensity generated in a phase winding being determined, for example, by the pulse duty factor of the assigned pulse-width-modulated signal. Since the phase windings are coupled with each other, for example in a star or delta circuit, and the current intensities generated essentially depend on the voltage differences between the voltages supplied to the first phase winding terminals, a first phase winding terminal of a selected phase winding may be connected to a fixed potential, for example, to ground potential or to the higher voltage potential of a DC link, and the differential voltages between all phase winding terminals may be predetermined by suitably calculating and generating the voltages at the terminals of the remaining phase windings. As a result, the supply voltage which is applied to the first, selected phase winding does not have to be switched back and forth between the different levels, so that the switching losses generated by the switching operations may be eliminated with regard to this phase. Overall, the switching losses of the machine are thus reduced. As a result of the reduction in switching frequency, an improvement of the electromagnetic compatibility (EMC) is also achieved.
A method is known from US 20040080293 A1, in which a selected phase winding is temporarily connected to a constant electrical potential, while pulse-width-modulated signals are applied to the remaining phase windings, and the pulses thereof are phase-shifted with respect to each other.